selinux: retain const qualifier on string literal in avtab_hash_eval()

The second parameter `tag` of avtab_hash_eval() is only used for
printing.  In policydb_index() it is called with a string literal:

    avtab_hash_eval(&p->te_avtab, "rules");

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: slight formatting tweak in description]
Signed-off-by: Paul Moore <paul@paul-moore.com>